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A span represents a single operation in a trace. A span could be representative of an HTTP request, a remote procedure call (RPC), a database query, or even the path that a code takes in user code, etc.

For example:

A trace

Above, you can see a trace with various spans. In order to respond to /messages, several other internal requests are made. Firstly, we check if the user is authenticated. Next we check if their messages were cached. Since their message wasn’t cached, that’s a cache miss and we then fetch their content from MySQL, cache it and then provide the response containing their messages.

A span may or may not have a parent span:

Spans are identified by a SpanID and each span belongs to a single trace. Each trace is uniquely identified by a TraceID which all constituent spans will share.

These identifiers and options byte together are called Span Context. Inside the same process, Span context is propagated in a context object. When crossing process boundaries, it is serialized into protocol headers. The receiving end can read the Span context and create child spans.

A span consists of the following fields:


Resource URL
Span specs specs/Tracing/Span
Span proto definition proto/trace/v1/Span.proto